1. Field of the Invention
The present invention concerns a method and a device in order to automatically determine a perfusion with a magnetic resonance system. Moreover, the present invention concerns a magnetic resonance system designed with the inventive device as well as an electronically readable data medium encoded with programming instructions that cause the method to be executed by a computer.
2. Description of the Prior Art
MR perfusion methods (methods to determine perfusion by means of a magnetic resonance system) are used, for example, in order to measure a blood flow in various body regions, for example in the head (cerebral blood flow (CBF)). CBF is the volume of arterial blood (mL) which flows in 100 g of tissue per minute; in humans a typical value for CBF in the brain is
            60      ⁢                          ⁢      mL              100      ⁢                          ⁢      g      ×      min        .If the density of the brain is set near to 1 g/mL, CBF in humans is 0.6
      mL          mL      ×      min        ⁢          ⁢  or  ⁢          ⁢  0.01  ⁢          ⁢            s              -        1              .  With reference to a volume, the dimensions of the CBF are simply the reciprocal of the time, i.e. a rate constant that defines the supply of a tissue volume with arterial blood. CBF has no causal connection with the quantity of the blood within a volume (blood velocity). According to the prior art, ASL methods (“Arterial Spin Labeling” methods) are used during a determination of the perfusion by means of a magnetic resonance system, wherein the water contained in blood is shifted into a particular magnetic state (normally an inverted magnetization); in short: the blood is “labeled” in order to be able to differentiate these blood particles which flow into a considered volume element from other tissue in this volume element. MR images are thereby generated with a perfusion-sensitive image sequence, and MR control images are generated with a control imaging sequence. The perfusion information is thereby represented only by a slight alteration in an image contrast which is present between the labeled blood particles flowing into a region of interest (which labeled blood particles exhibit the particular magnetic state) and the tissue in this region from which the MR images are acquired. A perfusion signal typically exhibits an intensity in the range of only a few percent of the entire intensity of the corresponding MR image. The acquisition of images of a relative perfusion or of images for calculation of a quantitative perfusion is therefore prone to artifacts. For this reason, at present multiple MR images for acquisition of a slight perfusion signal must be generated over multiple minutes, from which a series of MR images characterized with perfusion information and control images (without perfusion information) result over time. An image with perfusion information and a corresponding control image are respectively generated in alternation.
In order to generate MR images in which the perfusion is shown from these images with perfusion information and the corresponding control images, according to the prior art the difference is taken between an image embodying perfusion information and its corresponding control image. The difference value thereby obtained is then averaged over the series of images. A scaling or calibration factor is determined in order to arrive at a relative or quantitative perfusion information in the resulting MR images.
The quality of the conventionally calculated MR images shown perfusion information is low since the methods for determination of perfusion according to the prior art are very error-prone, for example with regard to artifacts or other interferences.